In a multipath propagation environment such as mobile communications, intersymbol interference occurs due to delay waves. In order to reduce the influence of the intersymbol interference, in recent years, implementation of turbo equalization technology has been studied.
FIG. 1 is a functional block diagram showing an example of turbo equalization in a conventional single-carrier communications receiver.
In the turbo equalization, a replica signal corresponding to an interference signal is generated from a reference signal 123 and a transmission path characteristic (estimation result) estimated by a transmission path estimator 101. The interference is removed from a received signal 121 by cancelling the replica signal. A minimum mean square error (MMSE) equalizer 103 performs MMSE equalization processing on the interference-removed signal. A likelihood calculator 104 calculates a bit likelihood from the equalization output, and a de-interleaver 105 performs de-interleaving on the bit likelihood. A soft-in soft-out (SISO) decoder 106 performs error correction on the de-interleaved result. A soft estimation value calculator 110 calculates a soft estimation value from the bit likelihood interleaved by an interleaver 109, and feeds back the soft estimation value as the reference signal 123.
Further, in a frame section of a known signal, the known signal stored in a known signal memory unit 113 is used as the reference signal 123, and thus a data symbol is also used in demodulation processing as the reference signal. By repeating the above described process, the interference signal can be eliminated.
In general, since a soft estimation value cannot be obtained in the first loop of the turbo equalization, the soft estimation value becomes ‘0’ stored in a zero storage unit 111. Accordingly, an interference replica is not generated, so that an interference cancellation is not performed.
Further, the transmission path estimator 101 does not update its transmission path characteristic in sections other than the known signal section and holds the transmission path characteristic until a next update.
Patent Document 1: Japanese Patent Application Publication No. 2002-190744
Non-patent Document 1: Tadashi MATSUMOTO et al.(1), “Turbo Equalization: Fundamentals and Information Theoretic Considerations,” <online>, January 2007, The Institute of Electronics, Information and Communication Engineers, <Searched on Jan. 31, 2014>, Internet <URL http://www.ieice.org/cs/jpn/JB/PDF/2007/j90-b_1_1.pdf>
However, in a fast fading environment, the conventional technology described above cannot follow such fading because the transmission path estimation is updated only in a known signal section in the first loop of turbo equalization. Therefore, the performance of the bit likelihood and the equalizer output in the first loop under the fast fading environment is deteriorated, and the number of loops of turbo equalization increases in order to obtain the desired performance, which is problematic. As a result, an initial delay until information bits can be obtained increases, and the initial delay is maintained as it is even in subsequent processing.